In normal binocular vision, an individual changes focus and alignment in order to maintain visual clarity whether an object is far away or in the near field. In order to maintain binocular fusion while focusing, several mechanisms must work in concert, including accommodation, the accommodation convergence mechanism, and the fusional vergence mechanism.
Accommodation is a process by which the eyes change their optical power to maintain a clear image on a target as its distance varies. Accommodation involves a coordinated change in lens shape, pupil size, in order to clear the image. The accommodative convergence mechanism is the associated positive (inward towards nose) and negative (outward away from nose) movement of the eyes during accommodation. The fusional vergence mechanism is the positive or negative movement of the eyes that compensates for a convergence deficiency (e.g. excess/deficit in accommodative convergence) in order to maintain binocular vision. Convergence disorders may be due to, for example, a phoria condition, fixation disparity, and/or proprioceptive disparity, also a muscular palsy, or neurological problem.
Proprioceptive disparity is an imbalance between where the eyes are consciously focused and the nonvisual perception of where the object is located in space—which often varies with distance.
When a proprioceptive disparity exists, the brain compensates in order to maintain a clear image. For some individuals, proprioceptive disparity often results in, for example, headaches, eye fatigue, pain around the eyes, blurred vision, neck pain, dry eyes, and other general symptoms of asthenopia.
Convergence disorders have been treated using prism lenses. Prisms cause the eyes to converge or diverge, and are added to a lens in order achieve a convergence balance between the eyes. However, the introduction of a prism alone does not alleviate asthenopia symptoms associated with proprioceptive disparity. For example, orthophoric patients with age related accommodation (e.g. normal accommodation and convergence) may still present with asthenopia due to proprioceptive disparity. Thus, prism lenses alone are not entirely effective and there have not been significant advances toward a more complete solution for problems such as proprioceptive disparity. Thus, there is a need in the art for more complete solutions to the proprioceptive disparity problem.